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A definition of quality in an optical system, indicating that it meets the Rayleigh criterion. This is met when there is no more than a quarter of a wavelength of light between the wavefronts reaching the focal point from all parts of the aperture. However, the practical resolving power of a telescope of a given aperture is better than this theoretical limit, so diffraction-limited optics are not necessarily perfect.

Subjects: Astronomy and Astrophysics.

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